<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <!-- 外联样式 -->
  <link rel="stylesheet" href="theme.css">
  <!-- 内嵌样式 -->
  <style>
  .text p {
    color: red;
  }
  div p {
    color: blue;
  }
  #main p {
    color: green;
  }
  .container #main p {
    color:orange;
  }
  .btn {
    background: black;
  }
  </style>
</head>
<body>
  <div class="text">
    <p>Hello</p>
  </div>
  
  <div class="container">
    <div id="main">
      <p>Hello</p>
    </div>
  </div>
  <!-- 行内样式，少用 -->
  <button class="btn" style="background: pink;">Click</button>
</body>
</html>